1. Field of the Invention
This invention relates to a scanning optical system, and more particularly to a scanning optical system using a sine vibration mirror.
2. Description of the Prior Art
The sine vibration mirror herein used refers to a mirror having a rotational angle .phi. which may be expressed with respect to time t as follows: EQU .phi. = .phi..sub.o .multidot. sink.sub.1 .multidot. t (1),
the rotational angle .phi. being the angle through which the mirror is rotated with respect to its reference position in which the reflected light resulting from the light beam from a light source to the center of the mirror being reflected by the mirror is coincident with the optic axis of the focusing lens. A typical example of such sine vibration mirror is the high-speed galvanomirror. If the galvanomirror is to be vibrated at high speed, a high frequency current must be flowed to the coil of the galvanomirror. If it is desired to make the vibration of the galvanomirror linear, this may be accomplished by flowing a saw-tooth high frequency current to the coil, whereas a saw-tooth high frequency current is difficult to obtain and therefore, a sine form high frequency current which is readily available is usually flowed to the coil. When such sine form high frequency current is flowed to the coil, the vibration of the mirror assumes a sine form.
The vibration velocity of the scanning beam of the scanning optical system using such a sine vibration mirror is necessarily a non-equiangular velocity. Accordingly, the movement of the scanning beam on the scanned surface becomes non-linear. The scanning system in which the movement of the scanning beam on the scanned surface is non-linear has been inconvenient for application to the so-called laser beam printer in which a laser beam is modulated in accordance with equally intervaled signals and the modulated laser beam is caused to scan the recording surface by a scanning system to thereby record information on the recording surface or for application to the so-called bar gauge in which an object to be inspected is disposed within the amplitude of the vibration of the scanning beam so that the time during which the scanning beam is interrupted by the object is measured to thereby measure the length of the object. On such a ground, a technique is well-known for long in which a distortion lens is disposed between the mirror in a scanning system capable of providing a scanning beam of non-equiangular velocity and the surface to be scanned so that the movement of the scanning beam on the scanned surface is linearly corrected by the distortion lens. For example, U.S. Pat. No. 2,692,369 issued on Oct. 19, 1954 discloses a device using a distortion lens to correct the non-linearity of the scanning beam caused by a light cam.
Also, U.S. Pat. No. 3,573,849 issued on Apr. 6, 1971, U.S. Pat. No. 3,687,025 issued on Aug. 19, 1972 and U.S. Pat. No. 3,345,120 issued on Oct. 3, 1967 disclose devices using a y.sub.2 f.theta. distortion lens to correct the non-linearity of the scanning beam caused by a deflecting-reflecting surface of equiangular velocity.
The present invention applies such technique to a scanning optical system including a mirror vibrated in a sine form.